The present invention relates to a sintered uranium dioxide pellet having a large crystal grain diameter and to a method for making the UO.sub.2 pellet.
The UO.sub.2 pellet of this type usually is hermetically sealed in a zircalloy clad tube and used as nuclear fuel.
To date attempts to improve the combustion efficiency of nuclear fuel have been made in order to increase the longevity of nuclear fuel, thereby making possible prolonged consecutive operation of a light-water furnace or a high-speed nuclear reactor. The more efficiently the nuclear fuel is burned, the more the amount of fission product (FP) generated by the nuclear fuel pellet increases. Of the fission product, a gaseous one such as xenon (Xe) is diffused into the crystal grain boundary, instead of being solved into the matrix of the nuclear fuel pellet, to form a bubble. The bubble formation causes swelling of the pellet and increases its volume, which in turn causes stress on the clad tube. This may cause a pellet clad interaction (PCI). Also, FP gas diffused into the crystal grain boundary is finally discharged from the pellet and then increases the internal pressure of the fuel rod, so that thermal conductivity of the gap between the pellet and the clad tube is reduced.
In order to prevent not only the increase of PCI but also the decrease of thermal conductivity of the gap, numerous attempts have been made with a view toward making nuclear fuel pellets with a large diameter grain so that the FP gas can be confined in the pellet. In this way, although the emission of gas per se cannot be prevented, if the pellet is made with a large diameter grain, e.g., the crystal grain diameter thereof is doubled, the distance between the place where the FP gas is generated and the grain boundary is also doubled, with the result that the discharge velocity of the FP gas is decreased.
Heretofore, a number of references such as Japanese Unexamined Patent Publication No. 2-124494 have suggested a process for enlarging the crystal grain diameter of the UO.sub.2 sintered pellet. According to the description in the foregoing publication, the process for producing a nuclear fuel pellet for a nuclear reactor is characterized in that the process, after a reduction step of a reconversion process, includes the steps of mixing a portion of uranium dioxide powder, which had not been subjected to a grinding step, with a remaining portion of uranium dioxide portion, which had been subjected to grinding, until a uniformly mixed uranium dioxide powder is obtained, pressure forming the resulting mixture of uranium dioxide powder into a compact, sintering the compact under high temperature, thereby obtaining the nuclear fuel pellet. In this manner, the invention provides a process for producing a nuclear fuel pellet having a large crystal grain.
However, the foregoing process for producing the nuclear fuel pellet having a large crystal grain has a problem in that a sufficiently high quality product is not always obtained.